Like most large transit system, the Washington Metropolitan Transit Authority makes its real-time data available to third-party developers, with the expectation that they will provide and update smartphone bus-arrival apps. In December, however, one of the most popular apps, NextBus DC, suddenly died.

The immediate problem was that the firm providing arrival prediction data to the app developer suddenly stopped sending data. Riders were so distressed that 7,000 users (out of 30,000) emailed the developer.

The arrival information was still being collected and was still available on the WMATA website and by phone, text, and other apps. But NextBus DC users didn’t know this, and the fact that WMATA’s web-based system was also called Next Bus added to the confusion. Reports Greater Greater Washington, “many riders assumed that the actual bus location technology had stopped working, and WMATA did not know where its buses were.”

Press coverage of the failure discovered long-standing complaints about WMATA from app developers. These included allegations of errors in the data and lack of response from WMATA to developers. WMATA disputed the first complaint, but acknowledged the second, and pledged to improve.

A year ago, Francisca Rojas, research director at Harvard’s Transparency Policy Project, compared five major transit agencies’ approaches to open data for app developers. He ranked Portland’s Tri-Met and Boston’s MBTA as the best, while WMATA and New York’s MTA the worst. Among the comparison criteria was “developer relationship,” where WMATA scored as “weak.”

Local coverage also exposed a problem in the format of the “application programming interface,” or API, used by WMATA to allow apps access to its data. The NextBus failure was caused when a private API providing information went dead; the app developer could have switched to WMATA’s direct API, but that takes time and the app developer had little notice. Perhaps more important in the long run is that various transit providers use various APIs, making it difficult for an app to provide consistent arrival information even within a region. WMATA said it would be reviewing its API, with an eye toward standardization, over the next six months.